The present invention relates generally to estimation of channel conditions in wireless communication networks and, more particularly, to a method and apparatus for generating supplemental pilot symbols from data symbols to enable more accurate estimation of channel conditions.
In mobile broadband communication systems, a plurality of user terminals may share an uplink and/or downlink channel. A scheduler at the base station determines which user terminals shall have use of the channel at any given time. For example, the shared channel may be divided in the time domain into consecutive time periods. For any given time period, a scheduler determines which user terminals shall be allowed to transmit or receive data on the shared channel. The channel could also be divided in the frequency and code domains so that more than one user terminal can use the channel in any given time period.
One example of a shared communication channel is the High Speed Downlink Shared Channel (HS-DSCH) for High Speed Downlink Packet Access (HSDPA) in Wideband Code Division Multiple Access (WCDMA) systems. A user terminal with an HSDPA connection estimates the channel based on pilot symbols transmitted by the serving base station on the Common Pilot Channel (CPICH). The channel estimates are used to generate a Channel Quality Indication (CQI) that is reported to the base station for scheduling purposes. The base station uses the CQI reported by the user terminals for scheduling and for selecting the coding and modulation to use for downlink transmissions. The user terminals may also use channel estimates to compute impairment covariances and combining weights, which are used in a RAKE receiver to combine different delayed versions of the received signal to produce symbol estimates. Chip equalizer receivers may also use the pilot symbols to adapt filter coefficients in channel tracking filters.
Channel estimates produced based solely on the CPICH can sometimes be noisy. The main difficulty is that there are only a limited number of pilot symbols per time slot (10 for HSDPA) available for channel estimation and/or covariance estimation. When the estimates are noisy, the user terminal may lower the reported CQI, or the base station may use a value lower than reported, in order to maintain a desired block error rate (BLER) and avoid excessive retransmissions. In both instances, there is a corresponding reduction in system throughput. Increasing the number of pilot symbols would improve the reliability of channel estimates, and therefore result in more accurate CQI reports and hence increased system throughput. Additionally, the user terminal would gain from improved interference suppression. However, increasing the number of pilot symbols increases the overhead and reduces the resources available to carry user data.